The present invention relates to a method for enhancing the efficiency of recovering energy possessed by an exhaust gas from a blast furnace.
A large quantity of exhaust gas is discharged from a blast furnace. Since this exhaust gas has large quantities of thermal and kinetic energies, if the gas is discharged into open air as it is, large quantities of energies are wastefully lost.
Accordingly, there have heretofore been made various attempts to recover a part of such energies while converting it to electric energy. A typical instance of these conventional attempts will now be described by reference to FIG. 1 of the accompanying drawings. An exhaust gas discharged from a blast furnace 1 is introduced through a duct 2 into a dust precipitating system including a dust collector 3, a duct 4 and a venturi scrubber 5 and it is then introduced into a turbine 7 through a duct assembly 6 (which comprises ducts 6a and 6b as later to be described). In the turbine 7, the energy possessed by the exhaust gas is converted to an energy for rotating a turbine shaft 8, and by this energy, a generator 9 is rotated. Thus, the majority of the energy of the exhaust gas discharged from the blast furnace 1 is recovered in the form of an electric energy. The exhaust gas discharged from the turbine 7 is transferred through a duct assembly 10 (which comprises ducts 10a and 10b) and it is then passed through a second venturi scrubber (not shown) according to need. Then, the exhaust gas is discharged into a low pressure gas line where the pressure is maintained at a level approximating to the atmospheric pressure.
In practising this method, however, problems are encountered since the pressure or flow rate of the exhaust gas is not always kept constant, and it is desired to make improvements for solving these problems. These problems will now be described one by one.
The flow rate of the exhaust gas is frequently changed mainly by opening or closing of a bell for feeding intermittently a raw material to the top of the blast furnace or by charging of the low-temperature raw material. In order to eliminate this disadvantage, the following arrangement is usually made in the energy recovery method shown in FIG. 1.
More specifically, the duct assembly 6 is connected to the duct assembly 10 through a duct 11, and a septum valve 12 is disposed in the midway of the duct 11 and a throttle valve 14 is disposed in the midway of the duct 6b of the duct assembly 6 extending on the side of the turbine 7 from a junction point 13 of the duct 11 (accordingly, the duct extending from the venturi scrubber 5 to the junction point 13 is the duct 6a). Further, a throttle valve 16 is attached to the duct 10a extending from a connecting point 15 of the duct assembly 10 and the duct 11 on the side of the turbine 7 (accordingly, the duct extending downstreams from the connecting point 15 is the duct 10b). The degree of opening in the septum valve 12 is adjusted according to the pressure detected by an oscillator 17 for detecting the furnace top pressure, and 70 to 90% of the total flow of the exhaust gas is introduced into the duct 6b leading to the turbine and the remainder, namely 30 to 10%, of the total flow of the exhaust gas is flown into the duct 11 in which the septum valve 12 is disposed. Namely, the basic portion of the exhaust gas that is not influenced even by variations of the flow rate of the exhaust gas is flown into the turbine, and the remainder of the exhaust gas exceeding the above basic portion that is increased or decreased by variations of the flow rate is flown into the septum valve, so that the furnace top pressure might be maintained at a predetermined level by the septum valve. According to this method, however, no power energy is recovered from the exhaust gas passed through the septum valve. As means for overcoming this disadvantage and improving the energy recovery efficiency, there has been proposed a method in which a turbine having a capacity such that the maximum quantity of the exhaust gas dischargeable when the turbine is operated in the normal state can be flown into the turbine is used, the total flow of the exhaust gas is ordinarily flown into the turbine and only when a "blow-off" phenomenon is caused in the blast furnace or the turbine must be stopped for some reason or the other, the septum valve is operated.
This method, however, also involves a problem to be solved. More specifically, even when the quantity of the gas generated in the blast furnace is reduced to a level lower than the quantity of the gas generated at the normal operation state or when the blast furnace is operated while maintaining the operation efficiency especially at a low level, controls should be made so that the furnace top pressure might be maintained at a predetermined level.
In order to solve this problem, there is ordinarily adopted a method in which a governor valve 18 is mounted on the duct 6b as shown in FIG. 2 and the degree of opening of the governor valve 18 is adjusted according to the pressure detected by the oscillator 17 for detecting the furnace top pressure so that the furnace top pressure might be maintained at a predetermined level or higher. In practising this method, when the furnace top pressure is lower than the predetermined level even if the total flow of the exhaust gas is introduced into the duct 6b, the governor valve disposed in the duct 6b should be further throttled so that the furnace top pressure might be restored to the predetermined level or higher. According to this method using the governor valve, the loss by throttling of the governor valve is inevitably caused: The quantity of the recovered energy becomes reduced by the throttle loss. This reduction of the recovered energy is especially serious when the operation efficiency must be maintained at a low level over a long period. More specifically, at the low-efficiency operation, the amount of the exhaust gas discharged is reduced, and even in such case, the gas flow must be considerably throttled by the governor valve so as to maintain the predetermined furnace top pressure. Accordingly, the throttle loss cannot be neglected. Therefore, it is eagerly desired to establish an energy recovery method in which the loss of energy by the use of the governor valve can be remarkably reduced.